Fin keels and low water

[Dan Tribe]

Apologies for thread drift, it is slightly relevant.
Anyone who has had a clinker boat will remember the "chuckle" they make in the slightest ripple. I found it very soothing.
A number of times I woke up with a start when the wind dropped in the night.
"It's all gone quiet, we must be aground!"

 

[Vid]

My fin keeler takes the ground in soft mud with every tide and generally gets maybe 15cm above her lines before starting to heel.

Thank you for answering the question: that's the sort of information that I was looking for

 

[davidej]

I was on a friend's bilger that took the mud in Hamford Water. The result was a distinct heel which caused us to slide out of our berths in the middle of night.

 

[Kukri]

I think a lot depends on what is under the mud. The mud does not go down for ever. In the middle of a river or creek that dries at low water springs you usually find hard gravel that you can walk on quite easily. It is kept clear by the fresh water running down at low tide. The mud is to the sides.

In talking to marina managers in Essex and Suffolk I learned that they generally dredge to not more than two metres, and won’t contemplate digging a deeper “pocket” for an extra deep boat, because if they go any deeper they hit really hard stuff.

 

[johnalison]

I think a lot depends on what is under the mud. The mud does not go down for ever. In the middle of a river or creek that dries at low water springs you usually find hard gravel that you can walk on quite easily. It is kept clear by the fresh water running down at low tide. The mud is to the sides.

In talking to marina managers in Essex and Suffolk I learned that they generally dredge to two metres, and won’t contemplate digging a deeper “pocket” for an extra deep boat, because if they go any deeper they hit really hard stuff.

I do the dredging for Titchmarsh on a regular basis, to 1.5m.

 

[PeterWright]

I think the point is that the harder material is dug just once, when the harbour is first built. and this is generally done with a big machine, such as a dragline, working in the dry if possible. The original depth is dictated by matters such as wall design (stonewall, steel piling, blockwork or bare mudbank) and, most of all, funds available. Wjen most East coast yacht harbours were built, a 2m depth would cater for most yachts, so th extra cost of going deeper would not attract many extra customers.

This build process usually cut down into fairly hard boulder clay. Since a harbour works like a settling tank, much softer silt, carried in on the rising tide, settles to the bottom reducing the depth. This material is removed by maintenance dredging which is done with much smaller, usually floating machinery which will fit without taking out all of the pontoons. This is capable of removing the silt, but struggles once it gets down to the hard clay left by the original constructed depth and encounters the hard clay. Hence the reluctance to dredge a deeper berth for one boat.

Also, if you dig one berth deeper than the surrounding area, it will naturally silt up much quicker to match the rest of the basin it is in.

Peter.

 

[Kukri]

Thank you Peter Wright and Jonalison.

I got a bit side tracked trying to offer an example - the point I was trying to make is not to do with depths in marinas but rather the point at which your fin keel stops sliding gently into soft mud, and hits pretty hard boulder clay, whereupon your boat stops sitting upright and will take a list one way or t’other.

The Deben at Woodbridge and Foundry Creek at Walton on Naze both have hard gravel bottoms and I have walked along both, wearing cheap boots, pushing a dinghy (which could float but not with me in it) ahead of me.?

 

[johnalison]

Thank you Peter Wright and Jonalison.

I got a bit side tracked trying to offer an example - the point I was trying to make is not to do with depths in marinas but rather the point at which your fin keel stops sliding gently into soft mud, and hits pretty hard boulder clay, whereupon your boat stops sitting upright and will take a list one way or t’other.

The Deben at Woodbridge and Foundry Creek at Walton on Naze both have hard gravel bottoms and I have walked along both, wearing cheap boots, pushing a dinghy (which could float but not with me in it) ahead of me.?

I think that the Deben generally has a fairly hard bottom, presumably due to the faster currents generally. We used to anchor off the sailing club at Waldringfield, which I think was fairly hard, and the Rocks off the beach is well named. The Colne generally seems softer. One chap kept a 40ft fin-keeler on a drying mooring for a year that must have drawn the best part of two metres, and that stayed upright.

 

[Aquaboy]

There are a couple of Caterlinas at Pinmill with wing keels. One spent last summer on a half tide mooring, the loading on the keel bolts must have been horrendous every time the tide returned though very soft mud.

 

[johnalison]

There are a couple of Caterlinas at Pinmill with wing keels. One spent last summer on a half tide mooring, the loading on the keel bolts must have been horrendous every time the tide returned though very soft mud.

I assume that you mean as the hull is lifted. The maximum load would at least be in line with the bolts, uo to the displacement of the boat, say 5 tons, divided by the number of bolts, though obviously normally much less than this. I would have thought that 100Kg or so would be well within what a bolt could manage, speaking from a position of complete ignorance in such matters.

 

[Aquaboy]

If you stand in a soft spot in your wellies just think how much effort is required to retrieve them. Can't be good for any part of the yacht really. I suppose the only good thing is its a slow and gently increasing pull as the tide returns and the spot will have got nice and sloppy overtime.

 

[Kukri]

I assume that you mean as the hull is lifted. The maximum load would at least be in line with the bolts, uo to the displacement of the boat, say 5 tons, divided by the number of bolts, though obviously normally much less than this. I would have thought that 100Kg or so would be well within what a bolt could manage, speaking from a position of complete ignorance in such matters.

If the boat were being held down by the suction of the mud on the wing plates the tension on the bolts and the floors would exceed the displacement as soon as the lwl was submerged, I think?

 

[Aquaboy]

You had to be careful loading a Thames barge if she had dried out level in a muddy spot. They are of cause totally flat underneath so it would be purely suction holding her down. Seem to think I read about it happening to one alongside in Faversham

 

[johnalison]

If the boat were being held down by the suction of the mud on the wing plates the tension on the bolts and the floors would exceed the displacement as soon as the lwl was submerged, I think?

I think so, but it wouldn't increase massively at first. I don't know what the potential buoyancy of my boat is, but I imagine that it would be about 20 tons for a 10m boat. I rather hope that my bolts are capable of coping with this.